1. Field of the Invention
The present invention relates to a liquid discharge apparatus in which individual liquid chambers partitioned by partitions made by a piezoelectric material are formed, and a manufacturing method of the liquid discharge apparatus.
2. Description of the Related Art
Conventionally, as a liquid discharge apparatus, a liquid discharge head which emits droplets by changing pressure of ink in an individual liquid chamber to generate a flow of the ink, and thus discharging the ink from a nozzle has been popularized. In particular, a drop-on-demand head has been most popularized. Here, there are roughly two methods of applying the pressure to the ink, that is, one is the method of changing the pressure to the ink by changing the pressure in the individual liquid chamber in response to a driving signal supplied to a piezoelectric element, and the other is the method of applying the pressure to the ink by generating bubbles in the individual liquid chamber in response to a driving signal supplied to a resistor.
The liquid discharge head in which the piezoelectric element is used can be manufactured with comparative ease by machine-processing a bulk piezoelectric material. Moreover, the liquid discharge head of this type has an advantage that restriction of ink is comparatively small and thus inks of wide-range materials can be selectively applied to a recording medium. Based on this point of view, in recent years, an attempt to utilize the liquid discharge head for industrial applications such as manufacture of color filters, formation of wirings and the like is often made.
In this context, in the piezoelectrically-actuated liquid discharge heads to be industrially utilized, a shear mode method is often adopted. In the shear mode method, a phenomenon that a polarized piezoelectric material is shear-deformed when an electric field is applied thereto in its orthogonal direction is utilized. Here, the piezoelectric material to be deformed corresponds to a partition which is formed by processing and making an ink groove or the like on the polarized bulk piezoelectric material with use of a dicing blade. A pair of electrodes is formed on both the side faces of the partition to drive the piezoelectric material, and the liquid discharge head is finally constituted by forming a nozzle plate having nozzles thereon and an ink supply system (Japanese Patent Publication No. H06-006375).
The liquid discharge head which adopts the shear mode method can be manufactured with comparative ease. However, to obtain desired discharge speed, it is necessary to control the pressure to be applied to the liquid in the individual liquid chamber by shear-deforming the piezoelectric material with voltage (a potential difference) to be applied to both the sides of the partition constituted by the piezoelectric material.
In general, discharge performance of the piezoelectrically-actuated liquid discharge head is indicated by a relation between the voltage and the discharge speed, and it has been known that the discharge speed is proportionate to the voltage. To obtain the liquid discharge head which can achieve low power consumption and superior controllability for discharge speed, it is necessary to enable to discharge a droplet with low voltage and reduce a percentage of a change of the discharge speed to the voltage (hereinafter, called a voltage sensitivity).
Since the discharge speed is proportionate to the pressure to be applied to the liquid in the individual liquid chamber, it is possible to control the discharge speed by adjusting the pressure to be applied to the liquid based on kinds of piezoelectric material, and widths and heights of the partition and the individual liquid chamber. For example, to increase the pressure to be applied to the liquid, it is effective to enlarge a displacement volume of the individual liquid chamber by narrowing the width of the individual liquid chamber and/or heightening the height of the individual liquid chamber.
Incidentally, a relation between the displacement volume and the constitutions of the partition and the individual liquid chamber is expressed as follows. That is, if it is assumed that the displacement volume is ΔVol, a piezoelectric constant is d15, the height of the individual liquid chamber is H, the width of the partition is T, the voltage is V, and the length of the individual liquid chamber is z, then a relational expression ΔVol=(d15×H×z×V)÷(4×T) is given.
However, if it intends to enlarge the displacement volume over the entire longitudinal direction by changing the width of the partition and the height of the individual liquid chamber, a percentage of a change of the displacement volume to the voltage becomes large according to the above relational expression. Since the displacement volume and the pressure to be applied to the liquid are in a proportional relation, a percentage of a change of the voltage to the pressure to be applied to the liquid becomes large resultingly. That is, if it intends to increase the pressure to be applied to the liquid in the individual liquid chamber by simply adjusting the width and the height of the individual liquid chamber in order to discharge the droplet with low voltage, the voltage sensitivity of the discharge speed increases, and controllability of the discharge speed of the droplet deteriorates.
In particular, if the diameter of the nozzle is made small up to, e.g., 5 μm to 15 μm to discharge minute droplets, since the distance between the wall face of the nozzle and the center of the nozzle becomes close to each other, influences of viscosity resistance and surface tension become large, and flow speed of the liquid tends to concentrate on the center of the nozzle. Thus, it becomes difficult to cut off a liquid column formed from the nozzle to the discharge direction. Therefore, when the liquid column is cut off and thus the droplet is formed, since motion energy stored at the central portion of the nozzle is large, the discharge speed of the droplet is high. That is, by making the diameter of the nozzle small, the pressure to be applied to the liquid in the individual liquid chamber, i.e., the percentage of the change of the discharge speed of the droplet to the voltage to be applied to the pair of electrodes (the voltage sensitivity), becomes steep much more, and thus the controllability of the discharge speed of the droplets further deteriorates.
In line with this, the present invention aims to provide a liquid discharge apparatus which has improved controllability of the liquid discharge speed of the droplets.